fix UnionType.__or__ and add UnionType.__getitem__

[KotlinIsland]

UnionType.__or__/__ror__

from types import UnionType
from typing import _SpecialForm

def f(u: UnionType, s: _SpecialForm):
    x = u | s
    reveal_type(x)  # UnionType, is actually _SpecialForm (is actually _UnionGenericAlias)
f(int | str, Literal[1])

this is rather unfortunate due to NotImplemented semantics, which are underspecified at this time, very relevant:

• special case Any in righthand-able operators to be NotImplemented lenient DetachHead/basedpyright#1465

to work around this we add the known case _SpecialForm to the return type

and

x = int | int | int
reveal_type(x) # UnionType, is actually type[int]

to work around this we add the special case type to the return type, in the case of "unions are invalid as return types", then i would suggest UnionType | Any

UnionType.__getitem__

T = TypeVar("T")
A = int | list[T]

A[int]

it has __getitem__, the design was inspired by _SpecialForm.__getitem__ but suggestions are appreciated

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

freqtrade (https://github.com/freqtrade/freqtrade)
+ freqtrade/optimize/hyperopt/hyperopt_optimizer.py:405: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]

xarray (https://github.com/pydata/xarray)
+ xarray/namedarray/core.py: note: In member "chunk" of class "NamedArray":
+ xarray/namedarray/core.py:813: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/core/variable.py: note: In member "_in_memory" of class "Variable":
+ xarray/core/variable.py:435: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/core/variable.py: note: At top level:
+ xarray/structure/alignment.py: note: In function "deep_align":
+ xarray/structure/alignment.py:993: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/structure/merge.py: note: In function "merge":
+ xarray/structure/merge.py:1012: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/core/resample_cftime.py: note: In member "__init__" of class "CFTimeGrouper":
+ xarray/core/resample_cftime.py:105: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/core/resample_cftime.py: note: In function "_adjust_bin_edges":
+ xarray/core/resample_cftime.py:286: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/core/dataset.py: note: In member "_validate_indexers" of class "Dataset":
+ xarray/core/dataset.py:2622: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/core/dataarray.py: note: In member "chunk" of class "DataArray":
+ xarray/core/dataarray.py:1474: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/core/dataarray.py: note: At top level:
+ xarray/core/dataarray.py: note: In member "_binary_op" of class "DataArray":
+ xarray/core/dataarray.py:4885: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/tests/test_variable.py: note: In function "test_datetime_conversion":
+ xarray/tests/test_variable.py:3131: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/tests/test_cftime_offsets.py: note: In function "test_rollforward":
+ xarray/tests/test_cftime_offsets.py:1004: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/tests/test_cftime_offsets.py:1006: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/tests/test_cftime_offsets.py: note: In function "test_rollback":
+ xarray/tests/test_cftime_offsets.py:1055: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ xarray/tests/test_cftime_offsets.py:1057: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]

sphinx (https://github.com/sphinx-doc/sphinx)
+ sphinx/pycode/ast.py: note: In member "visit_Constant" of class "_UnparseVisitor":
+ sphinx/pycode/ast.py:137:37: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ sphinx/util/_serialise.py: note: In function "stable_hash":
+ sphinx/util/_serialise.py:22:24: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ sphinx/util/_serialise.py: note: In function "_stable_str_prep":
+ sphinx/util/_serialise.py:45:24: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ sphinx/util/nodes.py: note: In function "apply_source_workaround":
+ sphinx/util/nodes.py:201:13: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ sphinx/util/nodes.py: note: At top level:
+ sphinx/transforms/__init__.py: note: In member "apply" of class "ApplySourceWorkaround":
+ sphinx/transforms/__init__.py:244:33: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ sphinx/config.py: note: In function "is_serializable":
+ sphinx/config.py:68:26: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ sphinx/config.py: note: In member "match" of class "ENUM":
+ sphinx/config.py:92:30: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ sphinx/ext/autodoc/preserve_defaults.py: note: In function "_get_arguments_inner":
+ sphinx/ext/autodoc/preserve_defaults.py:109:22: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ sphinx/writers/texinfo.py: note: In member "visit_title" of class "TexinfoTranslator":
+ sphinx/writers/texinfo.py:641:31: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ sphinx/ext/apidoc/_extension.py: note: In function "_check_collection_of_strings":
+ sphinx/ext/apidoc/_extension.py:246:37: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]

archinstall (https://github.com/archlinux/archinstall)
+ archinstall/lib/general.py:71: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]

core (https://github.com/home-assistant/core)
+ homeassistant/components/tplink/sensor.py:322: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]

attrs (https://github.com/python-attrs/attrs)
+ tests/typing_example.py:257: error: Argument 1 to "instance_of" has incompatible type "UnionType | <typing special form> | type"; expected "type[Never]"  [arg-type]

Tanjun (https://github.com/FasterSpeeding/Tanjun)
+ tanjun/commands/slash.py:378: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ tanjun/commands/slash.py:1642: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ tanjun/commands/message.py:106: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ tanjun/commands/message.py:163: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ tanjun/commands/message.py:234: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ tanjun/commands/menu.py:173: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ tanjun/commands/menu.py:300: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ tanjun/commands/menu.py:520: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]

egglog-python (https://github.com/egraphs-good/egglog-python)
+ python/egglog/egraph_state.py:643: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ python/egglog/runtime.py:247: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]

zulip (https://github.com/zulip/zulip)
+ zerver/lib/validator.py:596: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ zerver/openapi/markdown_extension.py:257: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ zerver/middleware.py:402: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]

artigraph (https://github.com/artigraph/artigraph)
+ src/arti/internal/type_hints.py:146: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ src/arti/internal/type_hints.py:148: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ src/arti/internal/type_hints.py:168: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]
+ src/arti/types/bigquery.py:266: error: Argument 2 to "isinstance" has incompatible type "UnionType | <typing special form> | type"; expected "_ClassInfo"  [arg-type]